Arthur (Clive Owen) is portrayed as a Roman cavalry officer, also known as Artorius Castus, the son of a Roman father and a Celtic mother.
He commands a unit of Sarmatian auxiliary cavalry in Britain at the close of the Roman occupation in 467 A.D. Arthur is loyal to Rome and a devout Catholic, but follows the teachings of Pelagianism, which many consider heretical.
He and his men guard Hadrian’s Wall against the Woads, a group of native Britons who are rebels against Roman rule, led by the mysterious Merlin.
Arthur and his remaining knights expect to be discharged from their service to the Empire after faithfully fulfilling a fifteen-year commitment.
However, on the night when they are to receive their freedom, Bishop Germanus sends them on a final and possibly suicidal mission to rescue an important Roman family living north of Hadrian’s Wall.
Marius Honorius faces impending capture by the invading Saxons, who are invading from mainland Europe, led by their king Cerdic and his son Cynric.
According to Germanus, Marius’ son Alecto is the Pope’s favourite godson and may be “destined to be Pope one day”.
Though angry, the knights follow Arthur once more but only after he and Lancelot argue over Arthur’s deeply held beliefs.
At the remote estate, Arthur discovers that Marius has immured pagans, including a Woad named Guinevere (Keira Knightley) and a small boy named Lucan.
Arthur defies Marius and frees the captives before deciding to take everyone, along with Marius’ family, back to Hadrian’s Wall.
One night, Guinevere takes Arthur to meet with Merlin, the leader of the Woads and her father.
At first, Arthur thinks Guinevere has betrayed him, but Merlin has come in peace.
It is revealed in a flashback that Arthur’s mother had died in a Woad attack when he was a boy.
Arthur’s famous sword, Excalibur, had belonged to his father and marked his burial mound.
Arthur had pulled it from the mound in an effort to rescue his mother from a burning building.
Merlin suggests an alliance between the Woads and the Sarmatian knights against the invading Saxons.